1. Field of the Invention
The present invention relates to a dual range infinitely variable transmission and, more particularly, to an infinitely variable transmission which includes two power paths to a pair of planetary gearsets. The transmission utilizes a continuously variable speed mechanism in conjunction with a fixed ratio speed mechanism for the two power paths. A two-position jaw clutch means is utilized to switch between the single path high range and the dual path low range of the transmission. A reaction type clutch is utilized to lock the sun gear of the second planetary gear set to provide a positive neutral.
2. Description of the Prior Art
The typical automotive transmission is shifted in specific, discrete steps between a high-torque, low-speed mode for vehicle launch and a high-speed, low-torque mode for vehicle operation at highway speeds. In a manual transmission, shifting is accomplished by the engagement of gear sets. In an automatic transmission, shifting is accomplished by the controlled engagement of friction elements. As a result of the limitations of shifting in discrete steps, the most efficient vehicle operation can only be approximated. Automotive engineers have long recognized that efficiency would be improved if the transmission could be adjusted continuously between ratios in order to compensate for changing conditions. Such continuous adjustment would allow engine operation to approach maximum efficiency for any particular condition.
Continuously variable transmissions, or CVT's, are directed to such increases in efficiency. The typical CVT employs a continuously variable speed mechanism in the form of a variable pulley with a pair of flanges mounted on an input shaft such that at least one of the flanges is movable axially with respect to the other. A similar variable pulley is mounted on an output shaft. A flexible belt couples the pulleys to allow the transfer of torque between the shafts when one of the shafts is driven. When the pitch radius of one pulley is changed, the pitch radius of the other pulley is changed simultaneously in the opposite direction. As a result, the drive ratio between the input and output shafts is varied in a continuous, smooth manner within the ratio range of the speed mechanism.
Heretofore, efforts have been made to extend the range of the transmission drive ratio of the CVT beyond the ratio range of the variable speed mechanism, that is, to devise an infinitely variable transmission. One such effort utilized two power paths between the torque input source and the rotating output. One power path comprised the conventional variable speed mechanism described above with variable pulleys on the input and output shafts and a belt connecting the pulleys. The second power path comprised a pair of sprockets, one located on each of the input and output shafts, and a silent chain drivingly connecting the two sprockets. The silent chain drive provided a fixed ratio drive.
In this prior art infinitely variable transmission, a sun portion of a planetary gear mechanism was driven by the output shaft of the variable speed mechanism. A planetary carrier portion of the same planetary gear mechanism was driven by the output sprocket of the fixed ratio chain drive. The transmission output speed in the dual drive mode was increased by changing the belt ratio to decrease the sun gear speed. The direct drive mode utilized only the variable speed mechanism as the fixed ratio chain drive became free wheeling.
An inherent weakness in such a prior art transmission which used more than a single clutch is that, during shifting, both clutches may be simultaneously engaged or simultaneously disengaged. A potentially dangerous situation is encountered during such simultaneous engagement or disengagement.
There has therefore been a need for an infinitely variable transmission with dual power paths which utilizes a clutch mechanism to actuate the dual drive and the direct drive. Moreover, there is a need for an infinitely variable transmission which reduces the number of components utilized in the gear reduction system and yet provides the necessary gear reduction for the output. There is also a need for a control system for such an infinitely variable transmission which allows precise control of the shifting between power paths and the componentry.
The infinitely variable transmission of the present invention provides such a structure and control system.